Electroless or autocatalytic coating of dielectric (non-conductor) substrates is a well known process finding wide utility in the preparation of such diverse articles as printed circuitry arrays (e.g., PTH), automotive trim, decorative plating, mirrors, decorative silver spray and the like. Normal electroless coating processes generally involve an initial cleaning and/or etching of the substrate by physical or chemical means as to improve the adherence of the metallic coating. In addition, the etched substrate generally provides with improved wettability toward water. The etched surface is then catalyzed or sensitized by suitable catalytic composition and processes to provide a surface capable of electroless (chemical) plating initiation.
In the prior art the catalytic treatment generally encompassed the use of precious metals (e.g., palladium). More recently, compositions and processes utilizing non-precious metals have been disclosed suitable for electroless plating of dielectrics. The following U.S. patents disclose the prior art as applied to non-precious metals as well as precious metal catalysts for electroless or chemical plating processes. These patents are included herein by reference.
U.S. Pat. Nos. 3,993,491; 3,993,799; 3,993,801; 3,993,848; 3,958,048; 4,048,354; 4,082,899; 4,087,586; 4,131,699; 4,123,832; 4,136,216; 4,150,171; 4,151,311; 4,167,596; 4,180,600; 4,181,759; 4,181,760; 4,220,678; 4,224,178; 3,011,920; 4,273,804; 4,265,942; 4,261,747; 4,259,087; 4,259,376; 4,233,344; Also, British Pat. No. 1,426,462 is included by reference. The following U.S. applications also reflect the state of the art and they are included herein by reference:
U.S. Ser. No. 052,857 now U.S. Pat. No. 4,278,712; U.S. Ser. No. 056,622 now U.S. Pat. No. 4,282,271; U.S. Ser. No. 061,484 now U.S. Pat. No. 4,301,190; U.S. Ser. No. 106,916 now U.S. Pat. No. 4,355,083; and U.S. Ser. No. 204,495 now abandoned. The prior art demonstrates that in the utilization of colloidal compositions, particularly those bearing non-precious metals, elemental state, compounds, or alloys bearing the non-precious metals of catalytic metals have been utilized directly or indirectly for the catalytic sites capable of electroless plating initiation.
In some of the applications disclosed above, particularly in the electroless plating for printed circuitry (e.g., PTH processing), there appears to be a change in surface charge especially after certain of the etching steps (e.g., ammonium persulfate). Such modification in a surface charge may adversely affect the adsorption or absorption of the catalyst (or sensitizer) onto the dielectric substrate or any other substrate and consequently affect its ability for electroless (chemical) plating initiation and as well as the resulting uniformity of plating. Incomplete electroless plating can often lead to skip plating. Such consequences cannot be afforded in commercial practices. Accordingly, at times it is highly desirable to provide with a manner by which a simple and inexpensive modification may be adapted compatible with the process and material, thereby insuring increased catalytic adsorption through the inclusion of a "prewetting" step.
In experimentation with the above prior art, particularly those colloidal systems used in the preparation of non-conductors, or printed circuitry type substrates, it has been noted that at times a certain failure mechanism takes place which shortens the lifetime of the colloidal catalytic composition. For instance, in examining some of the enabling examples in U.S. Pat. No. 3,958,048 it is also noted that the initial dispersion disappears within several hours and becomes a true solution. By contrast, some of the examples shown in U.S. Pat. No. 4,265,942 appear to deteriorate with time through the formation of brown dispersion and ultimate lead to green product which has a tendency of settling. Though the exact mechanism by which this deterioration takes place is not well understood, it is highly desirable to provide means whereby the lifetime for such colloidal dispersion may be extended through the incorporation of special additives (i.e., activator).